The usage of _dl_hwcap and dl_hwcap_mask is confusing

[Roland McGrath]

> Why shouldn't glibc support SSE when it is the part of the hardware?
> Should a user application be allowed to use SSE when it is available?
> I think it should be (GL(dl_hwcap) & HWCAP_I386_XMM).

The purpose of _dl_hwcap_mask is to mask that value, controllable with
LD_HWCAP_MASK to request omitting hwcap-specific code as if the machine
didn't have that capability.  HWCAP_I386_XMM needs to be added to
HWCAP_IMPORTANT as well since it's now used.